1. Field of the Invention
The present invention is directed generally to Quantum Well Infrared photodetector Focal Plane Arrays (QWIP FPA""s) and, more particularly, to QWIP FPA""s that are capable of multicolor detection.
2. State of the Art
Multicolor infrared detection capability has wide applicability to a number of different applications including aerospace, medical, surveying, mining, and agriculture. For example, multicolor infrared detection can provide useful information during geographical surveys. Detection in at least two infrared spectral bands would permit differentiation between man-made structures and natural landscape in a geographical survey. Additionally, in medical applications, multi-color detection would permit improved thermal imaging of the human body for diagnostic purposes.
Multicolor infrared detection has been conventionally performed using a wide-band IR detector and an associated rotating mechanical multicolor filter wheel. The wide-band IR detector detects a broad range of incident wavelengths and the rotating filter wheel selects the desired wavelength that is to be passed to the wide-band detector. An exemplary color wheel system is disclosed in U.S. Pat. No. 5,300,780. Mechanical color wheel systems, however, suffer from a number of deficiencies in multicolor detection. Such systems generally are slow and bulky, require large amounts of power for operation, and have a limited life span. Additionally, color wheel systems tend to have poor photon collection efficiency.
To alleviate some of the known deficiencies of the multicolor filter wheel, quantum well photodetectors have been constructed that permit the detection of more than one spectral band. One such quantum well photodetector is constructed using asymmetric quantum well layers that permit, in accordance with known principles, multicolor detection with a varying applied bias. An example of such a detector is described xe2x80x9cSwitchable Bicolor (5.5-9.0 xcexcm) Infrared Detector Using Asymmetric GaAs/AlGaAs Multiquantum well,xe2x80x9d Appl. Phys. Lett. 61 (3), Jul. 20, 1992 by Martinet et al. Varying the bias applied across such a photodetector varies the responsivity of the detector to impinging infrared radiation of different wavelengths. The multicolor detection capability of a single asymmetric quantum well layer in a single detector, however, has limited spectral responsivity. A single quantum well layer is only capable of being xe2x80x9ctunedxe2x80x9d within a specific spectral band that is limited by material parameters (e.g., quantum well depth) selected when constructing the detector. A single asymmetric quantum well layer in a single detector therefore does not permit multicolor detection over a wide spectral band.
Responsivity across a wide spectral range is achieved in exemplary embodiments of the invention by constructing the quantum well layers in a plurality of portions of an array of quantum well detectors so that the layers in each of the plurality of portions are responsive to different nominal peak wavelengths. A common voltage bias can be applied across each of the plurality of portions of the array to xe2x80x9ctune,xe2x80x9d using the Stark effect, the detectors in each portion of the array to different peak wavelengths then every other portion of the array.
One exemplary embodiment of the present invention is directed to a photodetector array which comprises an array of photodetector structures and further includes a voltage bias means. The voltage bias means provides different voltages for biasing different groups of detector structures in the array.
An additional exemplary embodiment of the present invention is directed to a photosensitive device which comprises a first photosensitive layer and two or more photosensitive layers, where the photosensitive layers are formed adjacent one another. This exemplary embodiment further includes a means for modulating a bias voltage applied across each of said photosensitive layers, where the modulated bias voltage is selectively applied to each layer and varies spectral responsivities associated with each layer.
A further exemplary embodiment of the present invention is directed to a photodetector array which comprises a plurality of photodetector structures, each comprising: a first photosensitive layer; two or more photosensitive layers, wherein said photosensitive layers are formed adjacent one another; and means for modulating a bias voltage applied across each of said layers, wherein said modulated bias voltage is selectively applied to each layer and varies spectral responsivities associated with each layer.
A further exemplary embodiment of the present invention is directed to a method of detecting radiation comprising the steps of: receiving radiation of a first spectral band in a first portion of a detector structure; receiving radiation of a second spectral band in a second portion of said detector structure, wherein said radiation in said second spectral band first passes through said first portion of said detector before reaching said second portion of said detector; receiving radiation of a third spectral band in a third portion of said detector structure, wherein said radiation in said third spectral band first passes through said first and second portions of said detector before reaching said third portion of said detector; and selectively converting said radiation in said first, second, and third spectral bands into quantities of moving charges.
An exemplary embodiment of the present invention is also directed to a photosensitive device comprising: first means responsive to impinging energy of a first spectral band; second means responsive to impinging energy of a second spectral band, wherein said impinging energy of said second spectral band first passes through said first means before reaching said second means; third means responsive to impinging energy of a third spectral band, wherein said impinging energy of said third spectral band first passes through said second means before reaching said third means; and means for selectively processing quantities of moving charges supplied from each of said first, second, and third means.